1. Field of the Invention
This invention relates to methods for polishing optical fibers. More particularly, the present invention relates to a precision method for controlling the depth to which a grinding or polishing device abrades the surface of an optical fiber.
2. Description of Related Art
While the concept of optical conductors has been around for more than a hundred years, advances made within the past forty years or so have made them the preferred communications transmission medium today. This is because of their low-loss transmission, high-information-carrying capacity, small size and weight, immunity to electromagnetic interference, unparalleled signal security, and the abundant availability of the raw materials (i.e. ordinary sand) required to make them. While they are higher in cost than conventional electrical conductors, their benefits are believed to far outweigh the slightly higher cost. As a result, fiber optic communication lines are gradually replacing conventional electrical communication lines at an increasingly rapid rate. For example, it is estimated that from 1970 to 1998, over 100 million kilometers of optical fibers were installed worldwide.
One of the greatest benefits of optical fiber communication lines is their enormous data handling capacity. For example, using advanced transmission techniques, a single pair of copper telephone wires can be made to carry about two dozen simultaneous conversations. However, by the mid 1980""s, it was already possible to transmit more than 12,000 simultaneous conversations over a single pair of optical fibers. The first transatlantic fiber optic cable, completed in 1988, could carry 40,000 simultaneous conversations using just two pairs of optical fibers. Advances since that time have increased the capacity of fiber optic transmission lines even more.
Couplers, amplifiers, etc. have been developed for use with optical fibers. These components typically involve coupling two or more optical fibers together in such a way that light signals traveling in one fiber may be transmitted either partially or completely into another fiber. To accomplish this, it is necessary to remove a portion of the reflective outer layer of the fiber, and produce a facet on an end or side of the fiber where light energy may enter and/or leave the core of the fiber. Polishing a fiber on its side and exposing the inner structures allows one to access the energy propagating in the fiber. This is usually done by polishing the fiber with a polishing lap.
Optical fibers vary in diameter from about 0.05 millimeters (50 xcexcm) to about 0.4 millimeters (400 xcexcm), and their inner structures have diameters much smaller. Since these fibers are so small and difficult to manipulate, it is very difficult to accurately polish a facet on the side of the fiber. Several methods have been employed to measure and control the depth of polishing of optical fibers. Visual inspection using microscopes has been employed for inspecting polished fibers after polishing to verify their depth of polish. See, e.g. U.S. Pat. No. 4,431,260 to Palmer.
Another method is to monitor power loss in the fiber caused by out-coupling to the polishing solution. A lubricating and cooling polishing solution is normally used when polishing an optical fiber in order to achieve a smooth, flat, polished surface which provides the desired optical properties. As the fiber is polished and the polished surface increases in size and approaches the center of the optical conductor, a signal introduced into one end of the fiber can be measured at the other end to determine the power loss which is caused by out-coupling of the signal to the polishing solution. This power loss gives a measure of the size and depth of the polished surface, and can be used to determine when to stop polishing. See e.g. U.S. Pat. No. 5,136,818 to Bramson.
Still another method involves placing a collimated light beam tangentially incident to a flat on an optical fiber while the flat is being progressively polished. A portion of the incident light from the light beam is transmitted into the optical fiber, the amount of transmitted light being a function of the surface area of the optical flat. By measuring the intensity of the light transmitted through the fiber, one can obtain a measure of the size of the flat. See e.g. U.S. Pat. No. 4,630,884 to Jubinski.
Unfortunately, some prior methods do not actually control the depth of polishing, but merely view the results of polishing after the fact. Where a polishing machine is adjusted based upon visual inspection of a polished fiber, this may not ensure acceptable or repeatable results.
It has been recognized that it would be desirable to have a reliable method for securing an optical fiber and accurately controlling the depth of polishing when the surface of the fiber is polished with a polishing lap. It is also recognized as desirable to have a simple and effective method for measuring the depth of polishing on a microscopic scale.
The present invention advantageously provides a system for polishing an optical fiber, comprising: an optical fiber disposed upon a substrate, the fiber having an outwardly curved portion; an electrical conductor disposed over the apex of the outward curve of the optical fiber; and a voltage source connected to the electrical conductor. A polishing lap of a polishing machine is brought to bear upon and grind away the electrical conductor, such that the electrical conductor is severed at approximately the same moment the polishing lap contacts the apex of the curve of the optical fiber. A detector detects when the DC voltage in the conductor suddenly changes, indicating severance of the electrical conductor, which indicates that the polishing lap has just reached the top surface of the optical fiber. This position is called the reference point. The polishing lap is then caused to progressively polish the fiber until a desired depth is reached relative to the reference point, the depth of polishing being detected with an optical probe. The invention also provides a method of polishing an optical fiber using the described system.
In accordance with a more detailed aspect of the present invention, the substrate comprises a piece of fused silica, in which one or more grooves are formed in which the optical fiber and the electrical conductor are cemented. One of the grooves is formed with an outwardly curved bottom surface, such that when the optical fiber is placed therein it will assume the desired outwardly curved configuration in preparation for polishing.
In accordance with another more detailed aspect of the present invention, the detector may comprise a digital interface electrically connected to the electrical conductor, and configured for detecting a voltage change therein, and a computer connected to the digital interface and the polishing lap, and configured for receiving signals from the digital interface, and for controlling the functioning of the polishing lap. Based upon the detection of severance of the electrical conductor, the polishing lap may be caused to polish to a predetermined desired depth, whereupon the computer may cause the polishing lap to stop polishing.
In accordance with another more detailed aspect of the present invention, the electrical conductor may comprise a copper wire which is placed over the apex of the outward curve of the optical fiber. Alternatively, the electrical conductor may comprise a stripe of conductive paint which is applied over the apex of the curve of the optical fiber, and electrically connected to the detector.
Additional features and advantages of the invention will be apparent from the detailed description which follows, taken in conjunction with the accompanying drawings, which together illustrate, by way of example, features of the invention.